1. Field of the Invention
This invention relates to a method of controlling an injection molding machine which controls specified operational processes by variably controlling the rotation speed of a driving motor of a hydraulic pump.
2. Description of the Relevant Art
In general, in a hydraulic injection molding machine, a specified hydraulic actuator is driven by a hydraulic driving unit equipped with a hydraulic pump, based on which operational processes are controlled such as a measurement process and an injection process in a molding cycle. Also, when the operational processes are switched, depressurization is performed on the hydraulic actuator so that the switching is performed smoothly and quickly without the behavior of the previous operational process (residual pressure) affecting the next operational process.
Conventionally, known as a method of depressurizing a hydraulic actuator is a method of depressurizing an injection driving device in an injection molding machine disclosed in Japanese Patent Publication No. 1999(11)-34135. This depressurization method is characterized by the fact that, in an injection driving device wherein a flow control valve and a cartridge valve whose the opening/closing of which is controlled by a pilot valve are installed in a pressure pipeline connecting a hydraulic pump to the injection-side oil chamber of an injection cylinder, and a proportional pressure control valve is selectively connected by a switching valve to a relief valve installed in a branch pipeline between the hydraulic pump and the flow control valve and to a back-pressure valve installed in a branch pipeline between the cartridge valve and the injection-side oil chamber to control the operation pressures of the relief valve and the back-pressure valve, the operation pressure of the back-pressure valve is lowered by the proportional pressure control valve after completing the injection operation of the injection cylinder to depressurize the operation oil in the injection-side oil chamber of the injection cylinder.
However, the depressurization method (control method) of an injection driving device in the conventional injection molding machine has had the following problems.
First, even if the residual pressure of the injection-side oil chamber of the injection cylinder could be made zero, it is possible that the residual pressure of the whole circuit from the back-pressure valve to the hydraulic pump cannot be made zero (unload pressure). Therefore, switching from the injection process using the injection cylinder to the measurement process using a measuring motor (an oil motor) for example, the residual pressure of the hydraulic pump side affects the measurement process, which prevents smooth switching by inducing a shock pressure when switching the operations, also preventing an accurate and stable measurement process.
Second, the operational pressure of the back-pressure valve is lowered by the proportional pressure control valve. Because it is a depressurization method relying on the natural decrease of pressure, the time until the desired pressure is achieved becomes long due to the fluidity and the operation oil and the flow path resistance of the circuit, and the demand of switching the operational processes quickly cannot be sufficiently met.